Брожение началось утром.

• Брожение = the subject of the sentence, in the nominative singular. It means fermentation (or sometimes ferment in a broader sense).
• началось = the verb, past tense, neuter singular, from начаться (to begin, to start).
• утром = a time expression meaning in the morning.

So the structure is basically:

[Subject] + [Verb] + [Time]

= Fermentation started in the morning.

Because брожение is a neuter noun.

In the past tense, Russian verbs agree with the subject in gender and number:

• masculine: начался
• feminine: началась
• neuter: началось
• plural: начались

Since брожение is neuter singular, the verb must be началось.

The basic pair is:

• начать = to start something
• начаться = to start / to begin

Compare:

• Они начали процесс. = They started the process.
• Процесс начался. = The process started.

In your sentence, брожение itself is what began, so Russian uses начаться, not начать.

That is why you get началось rather than a form of начать.

Russian often uses the instrumental case for certain time expressions, especially parts of the day:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So утром is a very common idiomatic way to say in the morning.

English learners often expect something like в утре, but that is not how Russian normally says it.

Yes. Russian word order is flexible.

For example:

• Брожение началось утром.
• Утром началось брожение.

Both are correct. The difference is mainly in focus:

• Брожение началось утром sounds neutral: first naming the event, then saying when it started.
• Утром началось брожение puts more emphasis on when it happened.

So the original sentence is a normal, neutral word order.

Russian has no articles.

So брожение can mean:

• fermentation
• the fermentation
• sometimes a fermentation

The exact English translation depends on context.

Russian speakers understand definiteness from:

• context
• word order
• shared knowledge
• the situation being discussed

началось is from the perfective verb начаться, which presents the beginning as a completed event: the starting point happened.

That fits naturally with English started.

начиналось would be imperfective and has a different feel. It might be used when:

• describing a process in progress,
• setting a scene,
• or emphasizing repeated/habitual beginnings in some context.

For a simple statement that something began, началось is the normal choice.

Its most basic meaning is literal:

• fermentation
• brewing
• biological or chemical fermenting

But Russian can also use брожение metaphorically, for example about social unrest, agitation, or ferment in society.

In this sentence by itself, the most likely reading is the literal one unless context suggests something figurative.

The stress is:

• броже́ние
• начало́сь
• у́тром

So: броже́ние начало́сь у́тром

A rough pronunciation guide:

• брожение ≈ bra-ZHEH-nee-ye
• началось ≈ na-cha-la-LOS'
• утром ≈ OO-tram

A couple of useful notes:

• Unstressed о is reduced in pronunciation.
• The final сь in началось is soft.

Usually брожение is treated like an uncountable or mass noun when it means the process of fermentation.

So in many contexts it behaves like English fermentation rather than a fermentation.

That said, Russian can use singular abstract/process nouns even when English might choose a different structure. Here the singular is the natural form because it refers to the process as one phenomenon.

Yes, but the tone or nuance changes.

• утром = the most neutral and common way to say in the morning
• с утра = more like since the morning / from early morning / starting in the morning, depending on context
• поутру = literary or old-fashioned

So for a plain, standard sentence, утром is the best choice.

Because the noun брожение is already the subject.

English often uses pronouns like it, but Russian does not need an extra dummy pronoun. The sentence already has a clear subject:

• Брожение = what started

So Брожение началось утром is complete exactly as it is.